Allow system_server to set powerctl_prop

Otherwise we break "adb root && adb shell svc power reboot",
which has the side effect of killing all of our test automation
(oops).

Bug: 11477487
Change-Id: I199b0a3a8c47a4830fe8c872dae9ee3a5a0cb631

system_server.te

@@ -148,6 +148,7 @@ allow system_server anr_data_file:dir relabelto;
 allow system_server system_prop:property_service set;
 allow system_server radio_prop:property_service set;
 allow system_server debug_prop:property_service set;
+allow system_server powerctl_prop:property_service set;
 
 # ctl interface
 allow system_server ctl_default_prop:property_service set;